Intracranial inoculation of susceptible mice with the JHM strain of mouse hepatitis virus (JHMV) results in acute and chronic encephalomyelitis and demyelinating disease, so it has been used as a model of multiple sclerosis (MS). No cure currently exists for patients with MS. Recently, it has been found that increasing disability in MS is correlated with axonal damage. The overall objective of this project is to characterize axonopathy and distinguish which cell types contribute to axon damage in JHMV-infected mice in order to identify potential therapeutic targets to allay axonal damage and disability. We hypothesize that early axonopathy is concomitant with axonal mitochondrial morphologic, transport, and functional changes in JHMV. One proposed cause of mitochondrial damage is reactive species released by activated macrophages and microglia. In post-mortem MS central nervous system (CNS) tissue, increased activated microglial and macrophage density and apposition has been correlated with axonal damage. Recent evidence shows that peripheral macrophage depletion reduces axonal damage in the experimental autoimmune encephalomyelitis murine MS model, while other work has shown that microglia may play an anti-inflammatory role in JHMV. Neutrophils are early effectors that release reactive species and contribute to recruitment of macrophages and lymphocytes. Therefore, we hypothesize that neutrophils and macrophages contribute to axonal damage. We hypothesize that (1) axonal and mitochondrial damage precedes adaptive-immune-mediated demyelination and (2) depletion of peripheral myeloid cells will mitigate axonal damage in the JHMV model of demyelination. We propose the following project aims to characterize axonal damage and delineate mechanisms in demyelinating disease. Aim 1: Characterize axonal mitochondrial damage prior to immune- mediated demyelination in a viral model of demyelination. This aim will characterize gross morphologic damage to axons as well axonal mitochondrial damage following CNS infection with JHMV. In particular, live two-photon imaging and metabolics phenotyping will be utilized to evaluate axon and axonal mitochondrial damage in JHMV-infected immunodeficient RAG1-/- and immunocompetent animals early after infection in order to delineate the contribution of innate versus adaptive immunity in axonopathy and in order to quantitate the time course of axonal damage in relation to demyelination. Aim 2: Elucidate contribution of resident versus peripheral myeloid cells in axonal damage in a viral model of demyelination. Two-photon imaging and whole organ clarification will be utilized in order to immunolabel and quantify juxtaposition of myeloid cells and axonal damage in JHMV-infected mice. Flow cytometry immunophenotyping will be performed in immunodeficient RAG1-/- and immunocompetent mice early following JHMV infection. The relative contributions of various myeloid cells to axonopathy will be investigated in animals experimentally depleted of microglia, macrophages, or neutrophils in advance of and during CNS infection with JHMV.